From rain to recharge: insights into field scale soil moisture and water flux dynamics at the Potsdam Soil Moisture Observatory (PoSMO)

Brandenburg is one of the driest regions in Germany and is highly dependent on groundwater resources for both drinking water supply and the growing demand for irrigated agriculture. Groundwater levels across the state are declining, and climate change is expected to further exacerbate this trend. The groundwater recharge rate (GWR) is a key parameter for the sustainable management of groundwater resources. However, its quantification it remains challenging, as it cannot be measured directly at spatial scales relevant for hydrological units in a landscape context.

In this study, we use daily data from several cosmic-ray neutron sensors (CRNS), which provide non-invasive measurements of soil moisture in the near-surface root zone at the hectare scale, to calibrate the soil hydrological model HYDRUS-1D. This calibration yields scale-effective soil hydraulic parameters and allows us to derive the downward water flux below the root zone as an approximation of GWR at the field scale.

The analysis is based on a unique six-year data set from the Potsdam Soil Moisture Observatory (PoSMO), a densely instrumented cluster located at and around an agricultural research site. The approximately 10-hectare area comprises multiple agricultural plots and extends along a gentle slope towards a lake. It is situated above a Pleistocene unconfined aquifer, with a groundwater table depth of 1 to 10 meters. At the heart of the instrumentation are eight continuously operated CRNS in combination with more than 25 point-scale soil moisture profiles measuring at depths of up to 1 m. A variety of additional measurements, including soil texture, hydraulic properties, continuous soil moisture measurements at depth, and groundwater level monitoring, provide a solid basis for validating the model and recording the relevant hydrological processes at the site.

In various simulation experiments, we evaluate the added value of using different soil moisture products for model calibration. To analyze long-term trends and fluctuations in GWR, we drive the calibrated model with historical weather data from over 50 years. We investigate changes in GWR under different climatic conditions and discuss the associated uncertainties, particularly in relation to the site's scarce water balance.